Restricted Access Phased Array (RAPA) is a short distance, corrosion scanning solution with a wide range of applications.
Restricted Access Phased Array (RAPA) is an advanced non-destructive testing technique that has a wide range of applications. This highly versatile inspection technique is used for the screening of corrosion in situations such as:
- Storage tank annular plates whilst the tank is still in service.
- Corrosion under support on pipelines.
- Pipelines entering equipment such as furnaces.
- Piping or structures where they enter the ground (air to ground interface).
Advantages of RAPA
RAPA is an efficient semi-quantitative screening method which produces similar results to the Short Range Guided Wave (SRGW) technique. The specific RAPA advantages include:
- Can detect shallow defects such as scallop and poor reflector corrosion.
- Can perform scanning with the plant or asset on-line, at temperatures below 100°C.
- The results can be seen in real time.
- Can provide accurate measurement of the corrosion area, providing length and breadth measurements, through encoded scanners.
- Minimal dead zone in front of probes compared to other techniques, such as Long Range UT.
- There are a wide range of difficult access applications.
- The inspections can be conducted using rope access solutions.
Scanning Tank Annular Plates
RAPA is an excellent technique for scanning the annular plate of above ground storage tanks from the outside of the tank whilst they are still in service.
Scanning the tank’s annular plates while they are in service allows an operator to understand any potential threats to the structural integrity of the tank and priorities inspections accordingly.
Prior to inspection some points that need to be taken into further consideration include:
- In most cases the annular needs to be cleaned to bare metal, removing coatings eliminates a method variable and improves signal transmission.
- The annular protrusion should be approx. 40mm or more (can be less in some cases).
- The tank can be in service and its contents will not affect the inspection.
- Scanning the tank's annular plates is an effective pre-shutdown inspection strategy aimed at ranking or prioritising inspection spend to where it is most needed.
Corrosion Under Pipe Support (CUPS)
Corrosion Under Pipe Supports (CUPS), or touch point corrosion, is a challenging issue for the integrity of pipework around many industrial sites, from power stations and mining to the oil and gas industry.
The touch point forms the perfect environment for corrosion to form and grow, and whilst it is often visible, it is difficult to inspect the area without lifting the pipeline or asset off the supporting structure. Such processes can be costly, present a safety hazard, and be logistically challenging.
ALS utilises RAPA to understand the corrosion that is occurring at touch points such as pipelines where they rest on supports. With RAPA, it is possible to pass the signal through the pipeline at the specific touch point area to effectively screen for corrosion. With RAPA, a basic severity assessment can be given using a “traffic light” system and is based on a signal amplitude assessment.
Common corrosion under support environments include:
- Clamped supports
- V shaped supports
- U shaped supports
- Pipelines resting on concrete supports
- Brace or bracket supports
- Sleeved type supports
Air to Ground Interface
Another inspection challenge particularly well suited to RAPA capability is the inspection of pipelines and structures at the point where they enter the ground. RAPA can penetrate the soil to scan for corrosion in the critical air to ground interface area.
Traditionally, it has been necessary to dig up the surrounding area in order to expose the structure for inspection. However, by using RAPA, this can be avoided which allows for an accurate but cost-effective assessment, and reduction in unnecessary excavations.
Using the same methodology applied to pipelines entering the ground, inspections can be performed on pipelines entering equipment. One such example of this is a pipe entering a heat recovery steam generator (HRSG).
Light Poles and Utility Poles
Considering the possibility for RAPA to detect corrosion at the air to ground interface, many utility companies are adapting this inspection technique to inspect large numbers of light poles and other utility poles at the area where they enter the ground.
When the results of this inspection technique are loaded into a dashboard software with a simple green (asset is okay), orange (asset needs monitoring), or red (asset needs to be repaired or replaced) interface it can be a very powerful tool for an asset manager with state or national responsibilities. This simple visual representation allows easy interpretation and understanding of multiple asset’s condition over a wide area and avoids unsafe or costly failures.
The ALS Approach
ALS continuously looks ahead and regularly invests in the latest technologies available to our industry. With continual advancement and superior customer value a priority, as well as comprehensive training and certification of technicians, ALS is able to continually deliver the most advanced, efficient and effective inspection methods available in the asset care field.
Technologies such as RAPA, which supplement our existing inspection expertise, are embraced by ALS only after successful method validation. This ensures a solid understanding of exactly where and how these technologies should be applied, and ensures all ALS inspectors are confident they will produce accurate results.